The present invention relates to measuring fluid composition and more particularly relates to a method and apparatus for measuring the concentration of a particular component in a two-component liquid flow stream.
There are many situations where it is necessary to determine the amount of a contaminant in a liquid stream. For example, in the oil industry, crude oil that is delivered through a gathering system to a pipeline or other terminal point normally is monitored with regard to its water content. This is routinely accomplished by means of a device commonly termed a "BS&W" or "water-cut" monitor which measures the concentration of water in the crude oil as the crude oil flows through a conduit. The water-cut monitors in most common use today are of the capacitance-probe type. The probe is inserted into the conduit and detects the presence of water by means of a change in the dielectric constant of the oil stream as it flows through the conduit and contacts the electrodes of the probe. This type monitor produces a read-out signal which is indicative of the percent water-cut, i.e. the concentration of water (and therefor of oil) in the crude oil stream at the instance that the measurement is made. Since the dielectric constants of most petroleum oils are in the nature of about 40 times as great as the dielectric constant of water, relatively small amounts of water may be detected by this type of monitor.
Available water-cut monitors of the type described above normally provide measurements of high accuracy and reliability when properly calibrated and operated under stable conditions. However, they are relatively expensive to use due to the sophisticated electronics involved. Also, they normally require a high caliber of preventive maintenance which may not always be convenient. Still more importantly, the readings of capacitance generated by presently known capacitance-type probes are directly related to the temperature and pressure of the liquid stream being monitored. This requires a particular capacitance-type probe to be recalibrated before almost every use to establish proper reference values, at the particular temperature and pressure expected to be encountered during the monitoring operation. Once calibrated for a particular temperature and pressure, a probe of this type will normally provide acceptable readings when minor temperature and pressure changes occur in the flow stream during a monitoring operation but is unable to maintain acceptable accuracy when major changes in temperature and pressure occur.